Refactor: split vnm.[ch] from vmod_netmapper.c

Also: s/ndb_t/vnm_db_t/, s/str_t/vnm_str_t/, etc..
Basic idea is to keep the Varnish-specific bits and
  pthreads/urcu stuff in vmod_netmapper.c, and move the
  generic database stuff (load, validate, lookup for database)
  to vnm.[ch] in generic C, where it can be shared with
  another tool for offline validation.

Change-Id: I19a72f46f90e100fe461412b99f16ee7e9f084c1